The disclosed subject matter relates generally to seals for turbine engines, and more specifically to abrasive coatings usable in such seals.
Typical sealing system for turbine engines refers to rows of rotating blades or knife-edge seals within a generally cylindrical case. The rotating elements—blade tips and knife-edges—are covered with protective abrasive coating while stationary elements have a relatively abradable surface; either the substrate itself is comparably abradable, or it can be covered with sacrificial abradable coating. To form a conventional seal, one of a pair of interacting sealing regions or edges are coated with abrasive material, while the other region is provided with an abradable surface as noted above. The abradable substrate or coating material is consumed in preference to the abrasive material and performs a cutting action into the abradable. Inadvertent contact can also cause a rub event, such as rubbing from differences in thermal expansion due to insufficient warmup periods, or other transient imbalances.
While the abradable material is consumed in preference to the abrasive material, the abrasive coating can, and is intended to, be consumed over time as well. In some instances, the matrix is intended to break down to release used abrasive particles and constantly expose new and sharper particles as particles are worn down through use. In addition, hard particles can pull out of the abrasive coating during certain rub events. Rather than damage the component substrate(s) during rub, pieces of both the abradable and the abrasive coating break off to limit rub forces that would otherwise be high during interference.
Given enough time and use, the entire abrasive coating could wear away, exposing the substrate. More frequently, however, the abrasive coating is usually intended to maintain a minimum thickness for sufficient sealing and protection of the substrate/cutting edge. In the case of compressor air seals, this minimizes surge (i.e., reverse flow over airfoil tips or other cutting edges).
During maintenance, there can sometimes be enough coating thickness to last until the next maintenance interval, perhaps due to less-than-expected wear. Since it is difficult to nondestructively inspect the seal for minimum abrasive coating thickness, most often, all of the blades are sent for refurbishment and recoating at predetermined intervals. This is conventionally done regardless of whether or not they have sufficient abrasive coating thickness to be reused for another interval.